282 



FUNDAMENTALS OF FRUIT PRODUCTION 



against freezing. How much the windbreak prevents the drying out 

 of the tree tops during the high, desiccating cold winds of winter is a 

 matter which with present data can be only conjectured. 



Certain experiments have shown that "with the temperature of the air at 

 84 and a relative humidity of 50 per cent, evaporation with the wind blowing at 

 5 miles an hour was 2.2 times greater than in the calm; at 10 miles 3.8; at 15 

 miles 4.9; at 20 miles 5.7; at 25 miles 6.1 and at 30 miles per hour the wind would 

 evaporate 6.3 times as much water as a calm atmosphere of the same temperature 

 and humidity. "*° 



Bates^^ found, in comparing wind movements in the open with those at a 

 leeward point distant from the windbreak five times its height, that "a wind 

 which reaches a velocity of 25 miles per hour in the open will, in the shelter of a 

 good windbreak, have a velocity of . . . only 5 miles per hour." 



Combining these sets of figures, the evaporation in this case would 

 be only 39 per cent, of that in the open. Figure 29, reproduced from 

 Bates' study, shows the percentage of protection to increase with the 

 wind velocity. 



100 



90 



80 



^10 



I 50 



^ 30 

 20 



to 



t) lu lb 21) 



"Wind Velocity in Open(miIes per hour) 



Fig. 29. — Wind at points five times the heights of windbreak to leeward, in terms of 

 wind in open. (After Bates^^) 



Effect on Evaporation. — Card^' in Nebraska determined the rates of evapo- 

 ration at different distances from a windbreak 8 rods wide and 25 to 40 

 feet high. Though these observations were made in summer, they are somewhat 

 indicative of winter conditions and furthermore they have an important bearing 

 on the state of trees as they approach dormancy. If the evaporation on the 

 windward side of the windbreak during all the time that drying winds were 

 blowing be represented by 100, then the evaporation at a point 12 rods distant 

 on the leeward side would be proportionally 83 and at 3 rods distant it would be 

 55. During a period of high though not particularly dry wind the respective 

 rates were as 100 to 67 to 29. Numerous interesting studies of evaporation 

 rates are reported by Bates, as shown in Table 25, arranged from his data. 



Effect on Soil Moisture. — The importance of soil moisture in relation 

 to winter drought has been shown. For this reason, Card's determi- 

 nations of soil moisture at varying distances on the leeward side of a wind- 



